ABSTRACT: Developing countries in the 'south' typically have a greater prevalence of entrenched dominant firms than economies of the large open economies of the north. This is due to various factors including scale and network economies relative to the size of the local markets, compounded by transport and logistics obstacles, the on-going influence of well-connected business groups and families, as well as the legacy of state support. The chapter takes Chile and South Africa as comparative case studies of how competition authorities have faced up to the challenges of assessing and addressing possible abuses of dominance. These countries have broadly similar economies in terms of the level of development, the importance of resource-based industries, and their relative isolation from other industrial countries. They are also similar in their competition institutions, with investigative bodies and independent specialist tribunals. Notwithstanding differences in the legal provisions, each has sought to apply effects-based tests to firm conduct – although with differences that have impacted on the outcomes. The chapter critically reflects on the record, against local expectations and the international debates, and provides concrete examples of the use of completion law in the telecommunication markets.